1. Field of the Invention
The invention relates to an AD converter.
2. Description of the Related Art
AD converters are available, for example, of successive and batch comparison types and can be further classified into a type using an integrator, that using a series resistor (generally referred to as “ladder resistor”) and so on. Brief description will be given below of an AD converter using a series resistor.
The AD converter has a series resistor made up of 2^m resistors (^ means “to the power of”) according to the resolution (e.g., m bits) of the converter. The series resistor is connected to a supply voltage VDD at one end and a ground voltage GND at the other end. A transfer gate (alternatively referred to as “transmission gate”; hereinafter referred to as “TG”), a gate made up of a parallel combination of a PMOSFET and an NMOSFET, is connected to each of connection points of the resistors making up the series resistor.
The TG functions as a so-called CMOS analog switch, which outputs the analog voltage of the connection point of the series resistor connected to the TG when both the PMOSFET and NMOSFET are brought into conduction as a result of application of voltages of the same level but opposite in polarity to the respective gate electrodes of the PMOSFET and NMOSFET.
The AD converter controls conduction/nonconduction of the TG and further converts, based on the result of comparison of the analog voltage (reference voltage) at a connection point of the series resistor output by the conducting TG and an analog value subject to be converted to a digital value, the analog value to a digital value. Refer to, e.g., Japanese Patent Application Laid-open Publication No. H06-120828.
Recent years have seen an accelerated transition to lower operating voltages in the semiconductor integrated circuits, and design and development efforts are underway to enable the AD converters and the TGs thereof to operate at low voltages. However, low-voltage operation of the TG causes the TG conduction current flow to become smaller, resulting in a problem of increased TG ON impedance. Further, due to the time constant, i.e., the product of the increased TG ON impedance and the wiring capacitance, the waveform of the reference voltage selected from the connection points of the series resistor becomes slow in transition, possibly degrading the AD conversion accuracy as a result of an erroneous comparison with the analog value before the reference voltage is fixed.
The problem of the reference voltage waveform being slow becomes particularly prominent when one half the supply voltage VDD (hereinafter referred to as “½VDD”) is selected as the reference voltage, that is, when gate-to-source voltages Vgs of both the PMOSFET and NMOSFET are in the vicinity of ½VDD.